Mexican authorities unexpectedly transferred the drug lord Joaquin "El Chapo" Guzman to a prison in Ciudad Juarez on the US border on Saturday morning, a source from the National Security Commission said.

Guzman's transfer, conducted under a large military security presence, was not connected to his possible extradition to the United States, the official said.

The transfer "strictly obeys federal penitentiary policy that is part of security protocols" to which some 7,400 prisoners have been subjected since last September, the commission said in a statement.

Guzman, who was arrested last January and held in a maximum security prison in Altiplano, 90 kilometers (55 miles) from the capital Mexico City, arrived at the airport in Ciudad Juarez at dawn, heavily guarded by some 150 federal police officers who had arrived earlier in three planes.

The transfer was connected to upgrades at the Altiplano prison, media reports cited Eduardo Sanchez, a spokesman for the president's office, as saying.

Meanwhile, Guzman's lawyer Jose Refugio Rodriguez, said Guzman's transfer is "illegal" because his defence team has launched "processes to stop him being transferred from one place to another."

A helicopter transferred Guzman from the airport to a prison in the south of Ciudad Juarez, across the border from El Paso, Texas.

Although not classified as a maximum security prison, it has a section for highly dangerous prisoners that is "one of the safest," the National Security Commission source said.

Military soldiers guard the prison, and the road leading to the facility is under especially heavy protection, Mexican media reported.

Watch | Re-captured 'El Chapo' arrives a maximum security prison

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The former leader of the Sinaloa drug cartel - one of Mexico's most powerful - Guzman staged a spectacular jailbreak in July 2015, when he escaped through a hole in his jail cell's shower that led to a 1.5-kilometer (one-mile) tunnel leading outside.

Considered one of the world's most powerful drug bosses, Guzman has twice escaped from maximum-security prisons. Mexican authorities began the process of deporting him shortly after his capture in January.

Mexican President Enrique Pena Nieto has said the extradition would take place "as soon as possible."